Transpiration by Trees

Transpiration Benefits For Urban Catchment Management

An analysis by Ted Floyd
First published in the Stormwater Industry Association Bulletin, no 92, Aug 2001

In Sydney a large gum tree transpires about 200 litres of water a day. This volume of transpiration will occur from a tree with 6m diameter of foliage on a hot summer day when the soil is still moist after recent rain.

Urbanisation of a catchment increases the area of impervious surfaces. Roads and buildings seal the surface which prevents water absorption by the soil. Impervious surfaces also prevent evaporation from soils.

Water Balance

1. Typical Undisturbed Catchment in Sydney Region

Interception and evaporation

from plant surfaces

15%

Runoff

15%

Seepage to groundwater and rivers

10%

Evapotranspiration

60%

2. High Density Urban Catchment

Runoff

up to 90%

Evapotranspiration and

seepage to ground water

10%

Evapotranspiration

Falling rain is intercepted by vegetation. A small proportion of the rain is evaporated directly from the plants surfaces. During rain, water is stored on the surfaces of leaves and stems. When it ceases to rain, water will continue to drip from a tree. This can help to even out a rainstorm and to reduce flood peaks.

Table 1 presents rainfall and evaporation data for Sydney. During the summer months evaporation is greater than rainfall. The difference between evaporation and rainfall is demonstrated by the P/E ratio. Yearly P/E for Sydney is 0.67 which demonstrates that evaporation is greater than rainfall.

During the growth of vegetation, water is removed from the soil by transpiration. The rate of transpiration is proportional to the leaf area. Trees have a large leaf area and deep roots encouraging a high transpiration rate. Transpiration by plants helps to dry out soils. During rain, water will infiltrate more readily into a dry soil. The removal of water by transpiration allows more water to enter soils during rain and this will reduce water runoff and lower flooding.

Table 1 RAINFALL AND EVAPORATION DATA SYDNEY

Monthly Rainfall - means (mm)

(Observatory Hill)

Jan

Feb

Mar

Apr

May

June

Jul

Aug

Sep

Oct

Nov

Dec

102

113

135

124

121

131

100

77

69

78

81

78

Yearly mean 1213mm

Monthly Evaporation (mm/month)

(Sydney Airport)

220

176

164

126

90

78

90

115

141

171

192

239

Yearly mean 1802mm

Daily Evaporation (mm/day)

7

6

5

4

3

3

3

4

5

6

6

8

P/E (precipitation/evaporation)

0.46

0.64

0.82

0.98

1.34

1.68

1.11

0.67

0.49

0.46

0.42

0.33

Yearly P/E 0.67

The rate of evapotranspiration is regulated by the total evaporation. Different plant species transpire at different rates. The ratio between evapotranspiration/total evaporation is often called the crop factor and can be as high as 0.95 for Lucerne in Jan. Deciduous trees vary from 0.75 in Jan down to 0.1 in June. When deciduous trees lose their leaves in winter they have a very low evapotranspiration rate. Table 2 lists values of crop evaporation factors for several crops.

Deep rooting plants have the ability to utilise water at greater depths in the soil. This enables these plants to grow and transpire when shallow rooting plants have wilted and ceased to transpire. Deep rooting plants generally have high crop factors, e.g. Lucerne develops very deep roots and has a high crop factor.

The depth roots penetrate into a soil depends on the availability of water. If no available water is in the subsoil, plants develop a shallow root system. Frequent light watering encourages shallow roots while less frequent heavy watering ensures water penetrates deep into the subsoil and encourages growth of deep roots. Plants with deep roots are more drought tolerant. Some Australian native trees have been observed with roots as deep as 30 metres. Table 3 lists the root depths of several common grasses.

In certain situations the removal of trees has resulted in the rise of the water table. This has caused great problems when there is salt in the subsoil. A rising water table brings the salt to the surface and this can kill vegetation and in urban areas salt can cause damage to buildings. In areas susceptible to salting, water infiltration into soils should not be encouraged and gardens and lawns should not be over watered.

The vegetation covering the land is a very important component of the water cycle. When vegetation is removed or reduced the cheeks and balances in the water cycle are disrupted. Trees are large, with deep roots having a major role in maintaining a balanced water cycle.